Organic light emitting device

ABSTRACT

An organic light emitting device including a plurality of organic layers between a first electrode and an emitting layer, wherein the organic layer includes an electron blocking layer. In one embodiment, a first organic layer, an electron blocking layer, a second organic layer and an emitting layer are formed on the first electrode. The electron blocking layer has a Lowest Unoccupied Molecular Orbital (LUMO) level which is lower than that of the first organic layer. Thus, the electron blocking layer traps excess electrons injected from the emitting layer, thereby improving lifetime characteristics of the OLED.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 10-2008-0014906, filed on Feb. 19, 2008, in the KoreanIntellectual Property Office, the entire content of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an organic light emitting device (OLED)including an anode electrode, an organic layer and a cathode electrodestacked on a substrate.

2. Description of the Related Art

Organic light emitting devices (OLEDs), which are self-emitting devices,have wide viewing angles, excellent contrast, and quick response, andthus have received a large amount of public attention. Because OLEDshave low operating voltage and quick response, and can realize multicolors, much research thereon has been carried out.

Typically, an OLED has an anode electrode/organic layer/cathodeelectrode structure. The organic layer functions as an emitting layer(EML). In addition, the organic layer may be formed of a plurality oflayers to further include functions of a hole injection layer (HIL), ahole transport layer (HTL) and an electron injection layer (EIL) as wellas the EML.

However, OLEDs cannot be easily applied to certain products becausereliability and lifetime (lifespan) characteristics of OLEDs are notsuitable for these products.

SUMMARY OF THE INVENTION

An aspect of an embodiment of the present invention is directed towardan organic light emitting device (OLED) having improved lifetime(lifespan) characteristics.

According to an embodiment of the present invention, there is providedan organic light emitting device (OLED) including: a first electrode, asecond electrode; an emitting layer formed between the first electrodeand the second electrode; an organic layer formed between the emittinglayer and the first electrode; and an electron blocking layer formedbetween the emitting layer and the organic layer and having a lowestunoccupied molecular orbital (LUMO) level which is lower than that ofthe organic layer.

The electron blocking layer may include a fullerene compound.

The organic layer may be a hole injection layer or a hole transportlayer.

According to another embodiment of the present invention, there isprovided an organic light emitting device (OLED) including: a firstelectrode; a second electrode; an emitting layer formed between thefirst electrode and the second electrode; a plurality of organic layersformed between the first electrode and the emitting layer; and anelectron blocking layer formed between the plurality of organic layers.

The electron blocking layer may not be in direct contact with the firstelectrode.

The plurality of organic layers may include a first organic layer formedon the first electrode and a second organic layer formed on the firstorganic layer, wherein the electron blocking layer is disposed betweenthe first organic layer and the second organic layer and has a lowestunoccupied molecular orbital (LUMO) level which is lower than that ofthe first organic layer.

The first organic layer may be a hole injection layer or a holetransport layer.

The second organic layer may be a hole injection layer or a holetransport layer.

The electron blocking layer may include a fullerene compound.

The OLED may further include a hole injection layer between the firstorganic layer and the first electrode, wherein the first organic layerand the second organic layer are hole transport layers.

The OLED may further include a hole transport layer between the secondorganic layer and the emitting layer, wherein the first organic layerand the second organic layer are hole injection layers.

The plurality of organic layers may include a first organic layer, asecond organic layer, a third organic layer and a fourth organic layerstacked on the first electrode, and the electron blocking layer mayinclude a first electron blocking layer formed between the first organiclayer and the second organic layer, and a second electron blocking layerformed between the third organic layer and the fourth organic layer.

The first electron blocking layer may have a lowest unoccupied molecularorbital (LUMO) level which is lower than that of the first organiclayer.

The second electron blocking layer may have a lowest unoccupiedmolecular orbital (LUMO) level which is lower than that of the firstorganic layer or the third organic layer, or is lower than that of theLUMO levels of the first organic layer and the third organic layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, together with the specification, illustrateexemplary embodiments of the present invention, and, together with thedescription, serve to explain the principles of the present invention.

FIG. 1 is a cross-sectional view schematically illustrating a structureof an organic light emitting device (OLED) according to an embodiment ofthe present invention;

FIG. 2 is a cross-sectional view schematically illustrating a structureof an OLED according to another embodiment of the present invention;

FIG. 3 is a cross-sectional view schematically illustrating a structureof an OLED according to another embodiment of the present invention;

FIG. 4 is a cross-sectional view schematically illustrating a structureof an OLED according to another embodiment of the present invention; and

FIG. 5 is a time-brightness graph illustrating lifetime characteristicsof an OLED according to an embodiment of the present invention.

DETAILED DESCRIPTION

In the following detailed description, only certain exemplaryembodiments of the present invention are shown and described, by way ofillustration. As those skilled in the art would recognize, the inventionmay be embodied in many different forms and should not be construed asbeing limited to the embodiments set forth herein. Also, in the contextof the present application, when an element is referred to as being “on”another element, it can be directly on the another element or beindirectly on the another element with one or more intervening elementsinterposed therebetween. Like reference numerals designate like elementsthroughout the specification.

FIG. 1 is a cross-sectional view schematically illustrating a structureof an organic light emitting device (OLED) according to an embodiment ofthe present invention. Referring to FIG. 1, the OLED according to thecurrent embodiment of the present invention includes a substrate, afirst electrode, a hole injection layer (HIL), an electron blockinglayer (EBL), a hole transport layer (HTL), an emitting layer (EML), anelectron transport layer (ETL), an electron injection layer (EIL) and asecond electrode. Here, in one embodiment, the ETL and/or the EIL mayoptionally and/or not be included in the OLED, if desired. In addition,according to the structure shown in FIG. 1, an organic layer, disposedbetween the emitting layer (EML) and the first electrode, includes theelectron blocking layer (EBL); and, more particularly, the EBL isinterposed between the hole injection layer (HIL) and the hole transportlayer (HTL).

The substrate may be any suitable substrate that can be used in OLEDs.In particular, the substrate may be a glass substrate and/or atransparent plastic substrate that has high mechanical strength, thermalstability, transparency, surface smoothness, can be easily treated, andis waterproof. A planarization layer and/or an insulating layer mayfurther be interposed between the substrate and the first electrode, ifrequired.

The first electrode is formed on the substrate, and may be patternedaccording to requirements of red, green and blue (R, G, B) sub pixels.The first electrode is an anode in the current embodiment, and may be atransparent electrode, a semi-transparent electrode or a reflectiveelectrode. A material used to form the first electrode may be ITO, IZO,SnO₂, ZnO, or the like, but is not limited thereto. In addition, thefirst electrode may have a structure having at least two layers using atleast two materials and/or other structures.

The HIL, which is an organic layer, may be selectively formed by thermalvacuum deposition and/or spin coating. The HIL may be formed of amaterial that can be suitably used to form a HIL. For example, thematerial used to form the HIL may be a phthalocyanine compound, such ascopperphthalocyanine disclosed in U.S. Pat. No. 4,356,429; a star-bursttype amine derivative, such as TCTA, m-MTDATA, and m-MTDAPB, disclosedin Advanced Material, 6, p. 677 (1994); a soluble and conductive polymersuch as polyaniline/dodecylbenzenesulfonic acid (Pani/DBSA);poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT/PSS):polyaniline/camphor sulfonic acid (Pani/CSA);(polyaniline)/poly(4-styrenesulfonate) (PANI/PSS); or the like, but isnot limited thereto.

For example, TCTA and m-MTDATA may be respectively represented byFormulae 1 and 2 below.

The EBL is formed on the HIL. The EBL has a lowest unoccupied molecularorbital (LUMO) level which is lower than that of the HIL. Accordingly,the EBL may trap excess electrons passing through the EML. Inparticular, the EBL inhibits excess electrons from flowing into the HIL,thereby improving lifetime characteristics.

The EBL needs to be formed of a material having a LUMO level which islower than that of the HIL. In this regard, the EBL may be formed of afullerene compound, for example, C60, C70, C76, C78, C82, C84, C90, C96,or the like.

Furthermore, the EBL may be formed of a derivative of the fullerenecompound having a substituent. The substituent may be a hydrogen atom, ahalogen atom, an alkyl group, an aryl group, an alkenyl group, ahaloalkyl group, a cyano group, an alkoxy group, a dialkyl amino group,a diary amino group, an aromatic cyclichydrocarbon group, or the like.

The halogen atom may be a fluorine atom, a chlorine atom, a bromineatom, or the like.

The alkyl group may be a straight-chain or branched-chain group havingfrom 1 to 8 carbon atoms such as a methyl group, an ethyl group, apropyl group, a hexyl group, an octyl group, or the like.

The aryl group may be an aliphatic cyclichydrocarbon group having from 7to 20 carbon atoms such as a benzyl group, a phenyl group, or the like.

The alkenyl group may have from 2 to 7 carbon atoms and may be anethylene group, a propylene group, a butylene group, a xylene group, orthe like.

The haloalkyl group may have from 1 to 4 carbon atoms and may be afluoromethyl group.

The alkoxy group may be a straight-chain or branched-chain group havingfrom 1 to 6 carbon atoms such as an ethoxy group, a butoxy group, ahexyloxy group, or the like.

The dialkyl amino group may have from 1 to 6 carbon atoms and may be adimethyl amino group, a diethyl amino group, or the like.

The diaryl amino group may have from 6 to 10 carbon atoms and may be adiphenyl amino group, or the like.

The aromatic cyclichydrocarbon group may have from 6 to 20 carbon atomsand may be a phenyl group, a naphthyl group, an anthryl group, aphenanthryl group, a pyrenyl group, a perylenyl group, or the like.

The aromatic cyclichydrocarbon group may have a substituent. Here, thesubstituent may be a halogen atom, an alkyl group, an aryl group, analkenyl group, a haloalkyl group, a cyano group, an alkoxy group, adialkyl amino group, a diaryl amino group, or the like.

For example, the C60 and derivatives thereof may include compoundsrepresented by Formulae 3 to 13 below.

In addition, the ETL may further include a material that further lowersthe LUMO level of the fullerene compound. For example, the fullerenecompound may further include an alkali metal.

Such an EBL may be formed using a wet coating method such as thermalvacuum deposition, spin coating, inkjet printing, spray coating and/ordip coating.

The HTL is formed on the EBL. The HTL, which is also an organic layer,may be formed by vacuum deposition, spin coating, casting, LangmuirBlodgett (LB) deposition, or the like. In one embodiment, vacuumdeposition is utilized because a uniform layer can be obtained and pinholes are prevented or reduced by the vacuum deposition. When the HTL isformed by vacuum deposition, conditions for vacuum deposition aresimilar to those for the formation of the HIL, although conditions forthe deposition and coating may vary according to the material that isused to form the HTL.

Examples of a material that can be used to form the HTL areN,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1-biphenyl]-4,4′diamine (TPD)and N,N′-di(naphthalene-1-il)-N,N′-diphenyl benzidine (α-NPD), but arenot limited thereto.

The EML is formed on the HTL. A material that is used to form the EMLmay be: a blue dopant such as oxadiazole dimer dyes (Bis-DAPOXP)), spirocompounds (Spiro-DPVBi, Spiro-6P), triarylamine compounds,bis(styryl)amine (DPVBi, DSA),4,4′-bis(9-ethyl-3-carbazovinylene)-1,1′-biphenyl (BCzVBi), perylene,2,5,8,11-tetra-tert-butylperylene (TPBe),9H-carbazole-3,3′-(1,4-phenylene-di-2,1-ethene-diyl)bis[9-ethyl-(9C)(BCzVB),4,4-bis[4-(di-p-tolylamino)styryl]biphenyl (DPAVBi),4-(di-p-tolylamino)-4′-[(di-p-tolylamino)styryl]benzene (DPAVB),4,4′-bis[4-(diphenylamino)styryl]biphenyl (BDAVBi), andbis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium III(FlrPic); a green dopant such as3-(2-benzothiazolyl)-7-(diethylamino)coumarin (Coumarin 6)2,3,6,7-tetrahydro-1,1,7,7,-tetramethyl-1H,5H,11H-10-(2-benzothiazolyl)quinolizino-[9,9a,1gh]coumarin(C545T), N,N′-dimethyl-quinacridone (DMQA), andtris(2-phenylpyridine)iridium(III) (Ir(ppy)₃); or a red dopant such astetraphenylnaphthacene (Rubrene), tris(1-phenyl-isoquinoline)iridium(III) (Ir(piq)₃), bis(2-benzo[b]thiophen-2-yl-pyridine)(acetylacetonate)iridium (III) (Ir(btp)₂(acac)),tris(dibenzoylmethane)phenanthroline europium (III) (Eu(dbm)₃(phen)),tris[4,4′-di-tert-butyl-(2,2′)-bipyridine]ruthenium (III) complex(Ru(dtb-bpy)₃*2(PF₆)), DCM1, DCM2, Eu(thenoyltrifluoroacetone)3(Eu(TTA)3 and butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran(DCJTB), but is not limited thereto. A light emitting polymer may be anaromatic compound including nitrogen and a polymer such as phenylene,phenylene vinylene, thiophen, fluorene and spiro-fluorene, but is notlimited thereto.

If desired, the EML may further include a light emitting host and alight emitting dopant. The light emitting host may be a phosphorescenthost or a fluorescent host. The fluorescent host may betris(8-hydroxy-quinorate)aluminum (Alq3),9,10-di(naphthyl-2-yl)anthracene (AND),3-tert-butyl-9,10-di(naphthyl-2-yl)anthracene (TBADN),4,4′-bis(2,2-diphenyl-ethene-1-yl)-4,4′-dimethylphenyl (DPVBi),4,4′-bis(2,2-diphenyl-ethene-1-yl)-4,4′-dimethylphenyl (p-DMDPVBi),Tert(9,9-diarylfluorene)s (TDAF),2-(9,9′-spirobifluorene-2-yl)-9,9′-spirobifluorene (BSDF),2,7-bis(9,9′-spirobifluorene-2-yl)-9,9′-spirobifluorene (TSDF),bis(9,9-diarylfluorene)s (BDAF),4,4′-bis(2,2-diphenyl-ethene-1-yl)-4,4′-di-(tert-butyl)phenyl(p-TDPVBi), or the like. The phosphorescent host may be1,3-bis(carbazole-9-yl)benzene (mCP), 1,3,5-tris(carbazole-9-yl)benzene(tCP), 4,4′,4″-tris(carbazole-9-yl)triphenylamine (TcTa),4,4′-bis(carbazole-9-yl)biphenyl (CBP),4,4′-bis(9-carbazoleyl)-2,2′-dimethyl-biphenyl (CBDP),4,4′-bis(carbazole-9-yl)-9,9-dimethyl-fluorene (DMFL-CBP),4,4′-bis(carbazole-9-yl)-9,9-bis(9-phenyl-9H-carbazole)fluorene(FL-4CBP), 4,4′-bis(carbazole-9-yl)-9,9-di-tolyl-fluorene (DPFL-CBP),9,9-bis(9-phenyl-9H-carbazole)fluorene (FL-2CBP), or the like.

The amount of the light emitting dopant may vary according to thematerial that is used to form the EML. However, typically, the amount ofthe light emitting dopant may be in the range from about 30 to about 80parts (or from 30 to 80 parts) by weight based on 100 parts by weight ofthe material used to form the EML (the total weight of the host and thedopant). In one embodiment, if the amount of the light emitting dopantis not within the above range, emitting characteristics of the organiclight emitting device may decrease.

The EML may be formed by vacuum deposition, spin coating, casting, LBdeposition, or the like.

The ETL is formed on the EML. The ETL may be formed of a material havinghigh electron-transporting capability. Examples of the material used toform the ETL include known ETL materials such as a quinoline derivative,in particular, tris(8-quinolinorate)aluminum (Alq3), TAZ, and Balq, butare not limited thereto. In addition, the material used to form the ETLmay be bis(10-hydroxybenzo[h]quinolinato beryllium (Bebq2) representedby Formula 14 below or a derivative thereof. The electron transportingmaterial may also include a metal oxide. The metal oxide may be an oxideof an alkali metal, an alkali earth metal or a transition metal.

The EIL is formed on the ETL. However, the EIL, which facilitatesinjection of electrons from the second electrode, may not be included.The EIL may be formed of LiF, NaCI, CsF, Li₂O, BaO, or the like.

The ETL and EIL may be formed by vacuum deposition, spin coating,casting, or the like. When the ETL and EIL are formed by vacuumdeposition, conditions for vacuum deposition are similar to those forthe formation of the HIL, although conditions for the deposition mayvary according to the material that is used to form the ETL and EIL.

The second electrode, as a cathode electrode, is formed on the EIL. Thesecond electrode may be formed of a low work-function metal, an alloy,an electrically conductive compound, or a combination thereof. In moredetail, the second electrode may be formed of Li, Mg, Al, Al—Li, Ca,Mg—In, Mg—Ag, or the like. In addition, the second electrode may beformed of a transparent material such as ITO or IZO to produce a frontsurface organic light emitting device.

The second electrode may be formed by vacuum deposition, sputtering, orthe like.

In the structure of the OLED having the EBL between the HIL and the HTLas described above, the EBL can trap electrons because it has a lowerLUMO level than that of the HIL. Thus, the EBL can prevent (or block)excess current passing through the EML from flowing into the HIL,thereby improving lifetime characteristics.

Hereinafter, various structures of OLEDs according to embodiments of thepresent invention will be described with reference to the arrangement ofa HTL between an EML and an electrode, and an EBL between a HIL and anorganic layer.

Here, the organic layers according to embodiments of the presentinvention may include HIL and HTL.

Materials for each of the layers and methods of manufacturing the layersare the same (or substantially the same) as those described withreference to FIG. 1, and thus descriptions thereof will not be repeated.Hereinafter, structures of the OLEDs which are different from thestructure of FIG. 1 will be described in more detail below.

FIG. 2 is a cross-sectional view schematically illustrating a structureof an OLED according to another embodiment of the present invention.Referring to FIG. 2, the OLED according to the current embodimentincludes a plurality of HTLs formed between a first electrode and anEML. That is, a first HTL is formed on the first electrode, and a secondHTL is formed on the first HTL. In addition, an EBL is formed betweenthe first HTL and the second HTL.

Since the EBL has a lower LUMO level than that of the first HTL, excesscurrent passing through the EML is inhibited from flowing into the firstHTL.

The first HTL and the second HTL may be formed of the same material ordifferent materials. When they are formed of different materials, thematerials may be appropriately selected in order to facilitate transportof injected holes to the EML.

FIG. 3 is a cross-sectional view schematically illustrating a structureof an OLED according to another embodiment of the present invention.Referring to FIG. 3, the OLED according to the current embodimentincludes a plurality of HILs and an EBL formed between the first andsecond HILs.

In more detail, a first HIL is formed on a first electrode, an EBL isformed on the first HIL, and a second HIL is formed on the EBL. Then, aHTL and an EML are formed on the second HIL. The HTL may not be formed,as desired.

Since the EBL has a lower LUMO level than that of the first HIL, excesselectrons injected from the EML are substantially trapped in the EBL.Thus, lifetime characteristics of the OLED can be improved.

FIG. 4 is a cross-sectional view schematically illustrating a structureof an OLED according to another embodiment of the present invention.Referring to FIG. 4, the OLED according to the current embodimentincludes a plurality of HILs, a plurality of HTLs and a plurality ofEBLs.

In more detail, a first HIL is formed on a first electrode, a first EBLis formed on the first HIL, and a second HIL is formed on the first EBL.Since the first EBL has a lower LUMO level than that of the first HIL,excess electrons are trapped in the first EBL.

Then, a second EBL is formed on the second HIL, and a first HTL isformed on the second EBL. Since the second EBL has a lower LUMO levelthan that of the second HIL, excess electrons are inhibited from flowinginto the second HIL.

A third EBL, a second HTL and an EML are sequentially stacked on thefirst HTL. Since the third EBL has a lower LUMO level than that of thefirst HTL, excess electrons are inhibited from flowing into the firstHIL.

Any suitable structure is within the scope of the present invention aslong as the EBL is interposed between the HTLs, and/or the HTLs areinterposed between the EML and the first electrode. However, the EBLshould not be in contact with the first electrode and needs to have alower LUMO level than that of an organic layer, such as a HTL or HIL,which is closer to the first electrode.

Hereinafter, the present invention will be described in more detail withreference to examples evaluating lifetimes of OLEDs according to thepresent invention. FIG. 5 is a time-brightness graph illustratinglifetime characteristics of an OLED according to an embodiment of thepresent invention.

An OLED according to an embodiment of the present invention(Experimental Example) and an OLED not including an electron blockinglayer (EBL) (Comparative Example) were manufactured in order to evaluatelifetime characteristics of the OLEDs.

EXPERIMENTAL EXAMPLE

An OLED having a structure described below was manufactured.

A corning 15Ω/cm² (1200 Å) ITO glass substrate (Corning) was cut intopieces of 50 mm×50 mm×0.7 mm in size, then the pieces were ultrasoniccleaned in isopropyl alcohol and deionized water for 5 minutes for eachof the pieces, and then the pieces were washed with UV and ozone for 30minutes to be used as a first electrode, i.e., an anode electrode. TheITO glass substrate was fixed in a vacuum deposition device.

Then, m-MTDATA was vacuum deposited to a thickness of 500 Å on the ITOsubstrate to form a HIL. C60 was vacuum deposited to a thickness of 50 Åto form an EBL. NPD was vacuum deposited on the HIL to a thickness of200 Å as HTL. Then, an EML was formed to a thickness of 400 Å by vacuumdepositing CBP as a host and PQ2Ir(acac) as a dopant on the HTL. Then,Alq3 was deposited to a thickness of 300 Å on the EML to form an ETL,and LiF was deposited on the ETL to a thickness of 5 Å to form an EIL.Then, Al was vacuum deposited on the EIL to a thickness of 1000 Å toform a second electrode, i.e., a cathode electrode.

COMPARATIVE EXAMPLE

An OLED was manufactured in the same (or substantially the same) manneras in Experimental Example, except that an EBL was not formed.

EVALUATION EXAMPLE Evaluation of Lifetime Characteristics of the OLEDs

Brightness of the OLEDs manufactured in Experimental Example andComparative Example was evaluated using a PR650 (Spectroscanspectrometer, PHOTO RESEARCH INC.), and the results are shown in FIG. 5.FIG. 5 illustrates brightness according to time when the initialbrightness of the OLEDs was regarded as 100%.

According to FIG. 5, when the brightness reliability was 90%, thelifetime of the OLED manufactured according to the Experimental Examplewas about 170 hours (a), and the lifetime of the OLED manufacturedaccording to the Comparative Example was about 95 hours (b). Thus, theOLED according to the Experiment Example of the present invention hasrelatively longer lifetime characteristics.

As described above, the OLED according to embodiments of the presentinvention includes organic layers (e.g., HIL and/or HTL) between the EMLand the first electrode, and an EBL between the EML and one or more ofthe organic layers (e.g., HIL). Thus, excess electrons passing throughthe EML are inhibited from flowing into the HIL, thereby improvinglifetime characteristics.

While the present invention has been described in connection withcertain exemplary embodiments, it is to be understood that the inventionis not limited to the disclosed embodiments, but, on the contrary, isintended to cover various modifications and equivalent arrangementsincluded within the spirit and scope of the appended claims, andequivalents thereof.

What is claimed is:
 1. An organic light emitting device (OLED)comprising: a first electrode; a second electrode; an emitting layerbetween the first electrode and the second electrode; a hole injectionlayer between the emitting layer and the first electrode; and anelectron blocking layer between the emitting layer and the holeinjection layer and having a lowest unoccupied molecular orbital (LUMO)level lower than that of the hole injection layer, wherein the electronblocking layer is directly on the hole injection layer.
 2. The OLED ofclaim 1, wherein the electron blocking layer comprises a fullerenecompound.
 3. An organic light emitting device (OLED) comprising: a firstelectrode; a second electrode; an emitting layer between the firstelectrode and the second electrode; a plurality of hole injection layersbetween the emitting layer and the first electrode; and an electronblocking layer between the plurality of hole injection layers, whereinthe electron blocking layer comprises a fullerene compound, wherein theelectron blocking layer is directly on one of the hole injection layers;wherein the plurality of hole injection layers comprise a first holeinjection layer on the first electrode and a second hole injection layeron the first hole injection layer; and wherein the electron blockinglayer is between the first hole injection layer and the second holeinjection layer and has a lowest unoccupied molecular orbital (LUMO)level lower than of the first hole injection layer.
 4. The OLED of claim3, wherein the electron blocking layer is not in direct contact with thefirst electrode.
 5. The OLED of claim 3, further comprising a holetransport layer between the second hole injection layer and the emittinglayer.
 6. The OLED of claim 3, further comprising a first hole transportlayer and a second hole transport layer, wherein the plurality of holeinjection layers comprise a first hole injection layer, and a secondhole injection layer, and the electron blocking layer comprises a firstelectron blocking layer between the first hole injection layer and thesecond hole injection layer, a second electron blocking layer formedbetween the second hole injection layer and the first hole transportlayer, and a third electron blocking layer formed between the first holetransport layer and the second hole transport layer.
 7. The OLED ofclaim 6, wherein the first electron blocking layer has a lowestunoccupied molecular orbital (LUMO) level lower than that of the firsthole injection layer.
 8. The OLED of claim 6, wherein the secondelectron blocking layer has a lowest unoccupied molecular orbital (LUMO)level lower than that of the first hole injection layer or the firsthole transport layer.
 9. An organic light emitting device (OLED)comprising: a first electrode; a second electrode; an emitting layerbetween the first electrode and the second electrode; a hole injectionlayer between the emitting layer and the first electrode; and anelectron blocking layer between the emitting layer and the holeinjection layer and having a lowest unoccupied molecular orbital (LUMO)level lower than that of the hole injection layer, wherein the electronblocking layer is a single layer, and wherein the electron blockinglayer is directly on the hole injection layer.